This invention relates to a mask holder for supporting a transfer mask for use in electron-beam exposure, ion-beam exposure, X-ray exposure and the like.
Recently, attention has been paid to electron-beam lithography, ion-beam lithography, and X-ray lithography as a method of manufacturing super fine device within a sub-half micron region, a quarter-micron region, or even less than 0.2 .mu.m for next generation. However, it is yet uncertain whether or not either of them will be suitable as a mass-production technique.
Among them, an exposure method, which is called a direct writing system (namely, one-drawing system), conventionally has come in practice with respect to electron-beam exposure using an electron beam.
In such a direct writing system, drawing operation is carried out by scanning an exposure pattern by the use of the electron-beam or the electron-ray having a small beam spot. In this method, it may be possible to make a super fine pattern.
However, this method has poor throughput because exposure time is excessively long. In consequence, this method is unsuitable for mass-production of a large integrated circuit (namely, LSI) or a very large scale integrated circuit (namely, VLSI) As a result, it has been assumed that this method is not preferable for the mass-production technique of the LSI or the VLSI.
Therefore, suggestion has been made about a drawing method which is called cell-projection exposure (this may be called block exposure) in recent years. In this cell-projection exposure system, the exposure is entirely and partially carried out by using various partial patterns (namely, component patterns) which repeatedly appear in the exposure pattern as masks. In this event, the exposure of a desired pattern is rapidly carried out by combining these various partial patterns (component patterns).
This method has short drawing duration (short writing time) and large productivity. Further, it is possible to make the superfine pattern in this drawing system.
In consequence, attention has been focused on this drawing method as the LSI manufacturing technique of the next generation.
In this event, a plurality of component patterns different from each other are normally formed on the transfer mask in this drawing method. In the exposure using this mask, the electron-beam is shaped by the use of the component pattern (namely, aperture), and a desired region (namely, block or cell) is partially and entirely exposed. When the transfer of one component pattern is terminated, the electron beam is deflected, and the transfer mask is moved to transfer the subsequent component pattern. The drawing is performed by repeating this operation.
Meanwhile, the transfer mask for a charged particle-ray exposure, such as, the cell-projection exposure due to the above-mentioned electron-beam, generally corresponds to a mask (namely, a stencil mask) having apertures.
In such a stencil mask, aperture patterns (namely, apertures) are formed so as to transmit the charged particle-ray for a thin-film portion supported by a supporting frame portion.
That is, there is no substance currently available which excellently transmits the charged particle-ray. Consequently, no substance can be interposed at a portion for passing the charged particle-ray. Therefore, this portion must be inevitably penetrated.
Moreover, when the aperture pattern is formed on a thick substrate, a passing charged particle-ray is adversely affected by a sidewall of the aperture pattern. Consequently, it is impossible to accurately transfer a pattern. Therefore, the portion (aperture pattern-forming region), in which the aperture pattern is formed, must be formed by a thin-film. Under this circumstance, the supporting frame portion having a preselected strength is necessary to support the thin-film with plane accuracy.
Such a transfer mask used for the cell-projection exposure has been conventionally fabricated by various methods. However, the transfer mask is generally manufactured by processing a silicon substrate (namely, a commercial silicon wafer and the like) using the known lithography technique or the known micro-machine processing technique from the viewpoint of processing performance or strength.
For instance, the supporting frame portion and the thin-film portion supported by the supporting frame portion are formed by etching and processing a back surface of the silicon substrate. Further, the transfer mask is fabricated by forming the aperture patterns in the thin-film portion.
In this case, an SOI (Silicon On Insulator) substrate, which is formed by laminating two silicon substrates via a SiO.sub.2 layer, is often used as the substrate. Herein, a silicon thin-film portion is formed by using the SiO.sub.2 layer as an etching stopper layer. For example, this method is disclosed in Japanese Unexamined Patent Publication No. Hei 6-130655.
In this event, it is difficult to handle the above transfer mask used for the cell-projection exposure because the transfer mask has small outline dimension between about 10 and 20 mm.quadrature.. Further, the transfer mask is generally manufactured by performing the micro-machine process for a silicon crystal substrate and the like, and the thin-film portion and the supporting frame portion are integrated to each other. Consequently, the transfer mask is extremely weak for external impact.
To solve these problems, the transfer mask is mounted to a mask holder formed by a metal conductor. Thereby, the handling becomes easy, and the transfer mask is protected from the external impact. Further, the mask holder, which mounts or supports the transfer mask, is attached to the electron-beam exposure apparatus, and allows heat to be radiated and electric charge to escape through the mask holder, thereby preventing accumulation of heat and charge. Herein, the heat and the electrical charge are generated for the transfer mask when the electron-beam is irradiated.
For example, a conventional mask holder that mounts the transfer mask has been disclosed in Japanese Unexamined Patent Publication No. Hei 9-92610.
In such a conventional mask holder, the transfer mask has a plurality of apertures. With this structure, the mask holder is arranged at an outer surface of the transfer mask except for the apertures.
However, the conventional example has the following problems.
First, the mask holder contacts with an upper portion and a lower portion of an aperture pattern. Therefore, it is required that a penetration hole (namely, an aperture hole pattern) is formed at a portion corresponding to the aperture in the mask holder. However, high accuracy is needed for the position and the shape of the penetration hole. In consequence, the formation of the penetration hole is complex, and the cost becomes high.
Even when the positioning accuracy and the shape accuracy of the penetration hole formed for the mask holder are satisfied, the aperture must be aligned with the penetration hole with high accuracy when the transfer mask is mounted to the mask holder. Consequently, the mounting operation becomes complicated.
As a result, the transfer mask often becomes useless because of mount defects occurring when an acceptable transfer mask, which is expensive and has difficulty in the fabrication, is mounted for the mask holder. This results in large loss.
Moreover, the mask holder contacts with the upper portion and the lower portion of the aperture pattern, and an aperture pattern forming region is formed by a thin-film portion (membrane). Consequently, distortion or twist takes place for the thin-film portion, and gives an adverse affect for transfer accuracy.
In addition, the mask holder contacts with the aperture pattern forming region of the thin-film portion. In consequence, the transfer mask is readily destroyed.